1. Field of the Invention
The present invention relates to a fixing device that heat pressure fixes a developed image on a record medium by use of a heated endless fixing belt, and a method thereof.
2. Description of the Related Art
In the prior art, as a fixing device which heat pressure fixes a toner image formed on a paper sheet and that is employed in image forming devices such as electronic photo type copiers, printers, and so forth, developed is a belt type fixing device wherein a toner image is heat pressure fixed by the steps of heating an endless fixing belt, and inserting a paper sheet into a nip portion between the fixing belt and a pressure roller that presses onto and contacts the fixing belt.
Conventionally, in this belt type fixing device of the prior art, as shown in FIG. 1, at the rotational center thereof, a fixing belt 4 is set between a heating roller 2, in which a heater 1 of electricity consumption of for example 600 W/h is built, and a supporting roller 3, and thereby the fixing belt 4 is heated by contact with the heating roller, while a pressure roller 6 is press contacted to a position facing toward the supporting roller 3 via the fixing belt 4, and a paper sheet 7 is made to go through a nip portion that is formed between the fixing belt 4, heated at a fixing available temperature by the heating roller 2, and the pressure roller 6, thereby the toner image is heat pressure fixed.
On the other hand, in electric photo type image forming devices in recent years, there is a demand for high speed image formation and a demand for reducing electricity consumption. Among components, especially a fixing device requires a heat generation source at high temperatures, and the electricity consumption of a fixing device occupies around 60% to 70% of the whole electricity that an image forming device consumes. Consequently, it is required to reduce electricity consumption of a fixing device to be employed in an image forming device without deteriorating high speed processing of the image forming device.
As a means to realize power saving by a fixing device, it is effective to reduce electricity consumption of a heat generation source by the fixing process. Besides this, saving power used by a fixing device is realized by shortening warm up time from the moment when an image forming device mainframe is turned on to the moment when the temperature of a fixing belt reaches a specified temperature, or shortening a first copy time from the moment when a next fixing process is in standby status after completion of a fixing process to the moment when the temperature of a fixing belt reaches a temperature for fixation availability.
However, in order to effectively save power in a belt type fixing device, and reduce electricity consumption of a heater as a heat generation source, and increase the temperature of a fixing belt to a temperature for fixation availability without deteriorating the high speed of image formation, it is required to increase a contact area where a fixing belt contacts a heating roller, and to sufficiently secure contact time of the fixing belt and heating roller.
One problem that remains, however, is that making the diameter of a heating roller greater, as a measure to sufficiently secure contact time between a fixing belt and a heating roller, leads to a larger size fixing device, that goes against aimed compact size and light weight of the fixing device. And further, a heating roller with a large diameter requires a large heat capacity to heat up the heating roller itself, as a result, warm up time and first copy time are made longer.
As a corollary, in belt type fixing devices, it is desired to effectively reduce electricity consumption, without deteriorating the compact size and light weight of the image forming devices, and without deteriorating the high speed processing of the image forming devices including shortened warming up time and first copy time.
Accordingly, an object of the present invention is to reduce electricity consumption in fixing process by a fixing belt without loss in a high speed of fixing time for a high speed image forming device.
Another object of the present invention is to reduce electricity consumption without making larger the size of a fixing device and an image forming device to which the fixing device is assembled.
Still another object of the present invention is to shorten warm up time and first copy time, thereby making fixing time faster, and reducing electricity consumption at warm up time and first copy time.
According to the present invention, provided is a fixing device comprising: first heating means, in the form of an endless belt, for heating a record medium having a developed image; pressurizing means, in the form of a roller, for pinching and conveying the record medium together with the first heating means to fix the developed image; second heating means, in the form of a roller contacting the first heating means for heating the first heating means at a specified temperature; and third heating means that is arranged in the second heating means and displaced from the rotational center of the second heating means to the first heating means side for heating the second heating means.
Further, according to the present invention, provided is a fixing device comprising: a fixing belt , in the form of an endless belt to heat a record medium having a developed image; a pressure roller, in the form of a roller to pinch and convey the record medium together with the fixing belt to fix the developed image; a heating roller, in the form of a roller contacting the fixing belt to heat the fixing belt at a specified temperature; and a heater that is arranged in the heating roller and displaced from the rotational center of the heating roller to the fixing belt side to heat the heating roller.
Still further, according to the present invention, provided is a fixing method wherein a record medium having a developed image is pinched and conveyed by a fixing belt which is heated by contact with a heating roller and a pressure roller, and thereby the developed image is heat pressure fixed, comprising the steps of: heating the heating roller by a heater that is arranged in the heating roller and displaced from the rotational center of the heating roller to the radius direction of the fixing belt side, and further displaced from the central position of contact area of the heating roller to the upstream side in the conveyance direction of the fixing belt; making the heating roller contact the fixing belt thereby heating up the fixing belt; and
inserting the record medium into a nip portion that is formed between the fixing belt after going away from the heating roller, and a pressure roller that pressure contacts the fixing belt.